Interesting Problem (?) with PWR THD vs FREQ Automated Test

QA40x
1

I am doing some testing on a 1969 vintage Kenwood Receiver. Since it has pre out/main in jacks, I decided to test the amp by itself. The receiver is rated at 53w/8ohms at 0.5%THD. I was able to run a THD vs Pwr and Freq test on the receiver as a whole with ok results (up to 40w/8ohms, vol set for 25dB gain). I set the amp up for 30w/8ohms- note that the gain is very high for a power amp- 46dB. Here is that plot:

30w
30w1202×889 199 KB

Note that the Gen level is -22.6dBV to get 30w. I then went into the automated test for PWR THD vs Freq and set up for 30w:

30w set
30w set360×484 23 KB

It came back with this:

set wrong
set wrong1176×876 200 KB

You can see that the gain it calculated is about 3dB less than what it measured previously and so it set the power up 3dB higher, which the Kenwood did not care for. I repeated the process (just the amp section) for 10w and 15w output power levels, no problems- worked as expected. Just starting at 30w… Thoughts?

2

It looks like the input range is overloading (36dBV). Does it successfully run if you check the autoset Input range checkbox?

image

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@restorer-john - As always, thanks for your thoughts on this. I rarely use the Autoset, just make sure I have enough attenuation for what I am running and my experience is that the autotests start out using the settings that you have for the main panel- to a certain extent, anyway. In this instance the attenuation for 36dB was there for the main panel- not sure what this particular test does as far as what level of attenuation it starts at. Right now I have the front panel off to clean the knobs and stuff, but when I get it back together I will try your suggestion and report back.UPDATE- I checked the Autoset Input range using 6dB since I was starting at 36dB (I assume) and it made no difference. It does measure the amp’s gain correctly, but does not set it correctly based on how I set it manually…

1 Like
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That’s interesting. I’ll try it myself over the weekend and see if I can get to the bottom of what it’s doing.

I am however running software version 1.223. I see you’re on 1.221, not sure if there’s any difference in respect of this test however.

I’ll post back my results. Cheers.

5

@restorer-john I am not sure why I am not usually using the latest release, but will do so a bit later today and post if it makes a difference. I will have the receiver for a few mores days before the owner picks it up. Interesting (to me) that the preamp section has a max gain of maybe -10dB and the amp is 47dB, not the norm for stuff I’ve looked at.

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Update- I ran using ver 1.223 and it did not make any difference. I did run it at 20w for comparison:

20wgd
20wgd1188×885 196 KB

and when I run the test I get this:

20wt
20wt1205×887 209 KB

which shows a starting point about 1dB higher than what I show it to require to get about 20w. I assume that it does it measurement at 1khz, but maybe not? This receiver’s frequency response is not flat so if it measured at another frequency that could explain the problem, but it is not bad enough to cause the unit to put out 70w when shooting for 20w…

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Deleted due to faulty thoughts..